This invention relates generally to methods and apparatus for linearization and spanning of transducer systems such as those using load cells and the like that have signal outputs versus input approach mathematical curves. More specifically, this invention relates to an improved transducer calibration method and associated apparatus for spanning and linearization of transducer data by employing an extremely small number of calibration data readings.
Transducers such as those that use strain gauges and other phenomena are well known in the art for providing output or data representative of an applied mechanical phenomena with nonlinear characteristics similar to a simple continuous curve. Load cells are included as one of the most common examples that apply; however, other phenomena such as heat transfer, gas diffusion, and the like might be included.
Inherent characteristics of load cells and/or transducers are well known to have outputs that are nonlinear over the useful range of the input. It has, therefore, been the practice to compensate for these nonlinear characteristics by either modifying the actual transducer output signal or by creating special transducer elements that have improved linear configurations. Most of these techniques are approximate at best and require substantial expenditures in time and money to accomplish.
For instance, approximate linearization of load cell data has been obtained through the use of circuit arrangements adapted to modify transducer output signals in a manner intended to compensate for the nonlinear characteristics inherent within the transducer. See, for example, U.S. Pat. Nos. 3,228,240 and 3,358,501. Adjustments of these circuits are done by trial and error at the expense of considerable calibration effort. Other linearization techniques have utilized programs that include lookup tables, best fit formulae such as the quadratic equation, linear interpolation, or cubic spline interpolation that modify the presented data. Solving these equations require the taking of many data points, and the solutions are approximations of best fit curves that can only be improved by taking additional data points. Likewise, the setting of the span during calibration is done by trial and error that also requires the taking of more data points.
Other linearization techniques, involving the shaping of the structure of the load cell element or the like, as described in U.S. Pat. Nos. 3,680,372 and 4,733,571, have been used. These techniques also require expenditure of additional time in complex machining.
Typically ten to thirty data points are taken during a calibration phase in order to provide sufficient information to define the characteristics of the transducer so that the final data can be presented to the required accuracy. Obviously, these efforts contribute substantially to cost and may only be partially acceptable where more accuracy is required.
There exists, therefore, a significant need for an improved method, including associated apparatus, for linearizing and spanning transducer data in a rapid and cost-efficient manner. The most cost-effective solution would be to reduce the number of data points taken during a calibration phase. The present invention fulfills these needs and provides further related advantages.